Photodynamic therapy (PDT), which can be used for many purposes including hair removal and acne treatment, involves the combination of a light-absorbing photosensitizing agent with light of the appropriate wavelength. A pre-photosensitizer, such as aminolevulinic acid (ALA, ALA-ester), which converts into a photosensitizing agent when it metabolizes, can also be used. The photosensitizing agent or pre-photosensitizing agent is topically or systemically applied to a target tissue where it accumulates. Upon irradiation with a visible light of an activating wavelength, the photosensitizing agent causes the release of reactive oxygen species in cells containing the photosensitizing agent, thereby promoting cell death.
While PDT induced using a photosensitizing agent or pre-photosensitizing agent has been somewhat successful, it can be difficult to control the treatment area. In particular, the photosensitizing agent or pre-photosensitizing agent can accumulate in healthy tissues as well as the target tissue. In hair removal, for example, the photosensitizing agent or pre-photosensitizing agent is applied to the skin topically and is absorbed by both the epidermal and dermal layers of the skin. As a result, application of light can cause phototoxicity to the epidermis, which can lead to long-lasting hyperpigmentation or epidermal necrosis.
Accordingly, there is a need for improved methods and devices for photodynamic therapy that reduce or eliminate damage to the epithelial tissue while allowing treatment of underlying targeted tissue.